This invention relates to electronic systems in which a resonant circuit is detected when brought into proximity to certain electronic equipment. Such systems are utilized particularly for security purposes, such as shoplifting prevention, but are not limited to such applications.
In their application to shoplifting prevention, such systems include equipment for establishing a radio-frequency (RF) field at the exit of, say, a retail store. Attached to each article of merchandise in the store which is to be protected from shoplifting is a tag bearing the resonant circuit, which is constructed to have a resonance frequency within the range of frequencies of the field. When the article is properly paid for, the clerk at the check-out counter either removes this tag, or else renders it effectively inactive by the application of a shielding label. Otherwise, the system senses the passage of the still-active tag through the RF field upon exiting and gives an alarm. For convenient reference, such systems are hereafter referred to as electronic article surveillance, or EAS systems.
The resonant circuit borne by each tag used with such EAS systems is a multilayer structure, having a dielectric substrate, on opposite sides of which are conductive layers so shaped as to define a capacitor and an inductor which cooperate to provide the circuit resonant at the desired frequency.
It has previously been proposed to render such a tag inactive by a more "elegant" technique than that of physical removal, or shielding. That improved technique is disclosed in U.S. Pat. No. 4,498,076, issued Feb. 5, 1985, in the name of George J. Lichtblau. It involves providing the tag itself with a localized region where the spacing between conductors on opposite sides of the dielectric substrate is reduced, e.g. by an indentation. It further involves providing electronic equipment which senses the presence of a tag (by a process generally similar to that used by the EAS system) and thereupon establishes a RF field at frequencies which include the resonant frequency of the tag and at a sufficient power level that breakdown occurs between conductors on opposite sides of the dielectric. This "deactivates" the tag and does so by purely electronic means.
In order to prevent confusion of terms between the EAS system previously described, and the electronic equipment used to sense and then deactivate the resonant circuit-bearing tags, the latter is referred to herein as an electronic deactivation, or ED system.
Deactivation using an ED system, in accordance with said U.S. Pat. No. 4,498,076, is a sound concept. However, there are matters of practical implementation which merit consideration beyond what is given to them in said Patent.
These include such items as how to avert possible interference between nearby ED systems, or between ED and EAS systems, how to provide suitable indications of tag deactivation, and how to dissipate the relatively high RF power which is developed by the ED system during deactivation.
It is an object of the present invention to deal with the matters noted above.